1. Field of the Art
The present invention relates generally to a wind tunnel and to a collector design therefor, and more particularly to an open jet wind tunnel with a collector design to reduce resonance in aerodynamic and acoustic testing applications. The invention also relates to a method of providing a wind tunnel and exposing test objects to the wind tunnel air flow for aerodynamic and acoustic testing.
2. Description of the Prior Art
An existing and well-recognized problem with many open jet wind tunnels is the pulsing or resonance which occurs at various frequencies under certain operating conditions. Open jet wind tunnels are used extensively in the automotive industry and various other industries for the purpose of determining aerodynamic forces on a test vehicle or other object and for the purpose of determining the effect of air flow on the aerodynamic and acoustic performance of a particular vehicle design. A typical configuration of an open jet wind tunnel includes a test section often comprising a large room or other closed configuration, a nozzle at the upstream end of the test section and a collector at the downstream end of the test section. Generated air flow flows along a flow path from the nozzle, across the test section and into the collector. During a test, the vehicle or other object to be tested is positioned in the test section within the air flow.
An observed problem with many open jet wind tunnels which often restricts their use at certain air speeds includes a pulsing phenomenon or resonance which occurs at such air speeds. This pulsing or resonance, in some cases, may simply make the air flow and the floor pressures unsteady or variable so as to adversely affect the air flow past the test object. This can result in inaccurate and thus unreliable data. In other cases, the pulsing or resonance may be strong enough to damage the building structure. Various authorities have recognized this problem in open jet wind tunnels and have speculated that such pulsing phenomenon or resonance is directly related to test section air speeds and wind tunnel dimensions and that the mechanism that excites such resonance involves the interaction of vortices of air flow from the nozzle to the collector. It has also been speculated that decreasing the length of the test section at a given air speed will have the tendency to reduce resonances at that air speed. However, this is a limitation on the capability of the system and is often not an option because of the test section length needed for a particular application. Accordingly, to the extent that this problem has been addressed, it has been primarily addressed by adjusting air speeds and/or wind tunnel and test section dimensions in an attempt to avoid the undesirable resonance problems. The result, however, is that this merely changes the frequency or air speed conditions at which the resonance or the pulsing phenomenon occurs or places other significant limitations upon the system.
Even when resonance problems have been addressed as provided above, utilization of a wind tunnel for providing air flow tests on a test object such as a vehicle involves providing a wind tunnel with a certain size, including nozzle size and test section length relative to the test vehicle, so that it accurately and reliably simulates road conditions for the particular vehicle performance parameter being tested. This is particularly critical when the wind tunnel is being used in the aerodynamic and acoustic testing of vehicles as discussed below.
Automotive designers and manufacturers utilize open jet wind tunnels to test vehicle performance in various performance areas. One such area involves evaluating or testing the effect of air flow on its ability to cool the vehicle engine. This is sometimes referred to as “climatic” testing or testing conducted in “climatic” wind tunnels. In climatic testing, the designer is concerned primarily with the cooling effect of air flow at the “A” pillar (the front) of the vehicle where the air enters the engine area or other air intake and is less concerned, if at all, with any aerodynamic or acoustic effect the air flow may have on the vehicle. Thus, a wide range of wind tunnel sizes (in terms of nozzle size relative to vehicle size and test section length relative to nozzle size) can be used for climatic wind tunnel testing on vehicles.
A second performance area involves evaluating or testing the aerodynamic and acoustic effect of the air flow as it flows past the vehicle. In aerodynamic and acoustic testing, the design of windshield wipers, the design of radio antennas, the overall exterior configuration of the vehicle from the front to the rear, the effect of open vs. closed windows, etc. are important. As a result, wind tunnels designed for use in, or used in, the aerodynamic and acoustic testing of vehicles requires careful design to ensure accurate simulation of road conditions at the applicable speeds and thus reliable design data. Thus, in contrast to wind tunnels used for climatic testing purposes, wind tunnels used for aerodynamic and acoustic testing purposes have heretofore required a relatively narrow range of wind tunnel sizes (in terms of nozzle size relative to vehicle size and test section length relative to nozzle size).
In general, for wind tunnels utilized in aerodynamic and acoustic testing purposes, the size of the nozzle opening and the distance between the nozzle and the collector (the test section length), and thus the overall size of the wind tunnel, is dictated by the size of the test object or vehicle. Specifically, to achieve accurate and reliable results and to avoid resonance problems for aerodynamic and acoustic testing, the consensus is that the “blockage” of the test object (the cross-sectional area of the test object in the air flow) should not exceed 10% of the nozzle opening and that the overall length of the test section should not exceed about three to three and one-half times the hydraulic diameter of the nozzle opening. Thus, a test object with a “blockage” of twenty-five square feet (25 sq. ft.2) requires a nozzle opening of at least about 250 ft.2 (and thus at least a hydraulic diameter of about 16 ft.) and an overall test section length no more than 48 to 56 feet for reliable aerodynamic and acoustic testing. In many cases, this significantly limits the length of the test object or vehicle that can be tested in that particular wind tunnel.
Accordingly, there is a need in the art for a wind tunnel construction, and in particular an open jet wind tunnels for aerodynamic and acoustic testing of vehicles or the like, which not only minimizes, but preferably eliminates, resonance problems for desired air speed and the wind tunnel dimensions, and which provides improved space efficiency and thus reduced capital and operating expenses.